Great planning and thought put into this! I wasn't aware of it all earlier in the thread, so thanks for the explanation.
Yes, I'd want my Pi Zero to have a jack in the back with separate power - to keep it running even when I turn the console off. Hard shutoffs for these things are hard to tell if you'll corrupt something - I have managed to corrupt Raspbian on a couple occasions where the kids yanked the RetroPi power out - and I just had to reimage it and move everything over again.
The Zero takes minuscule voltage anyway, so I don't mind leaving it on. I wonder if a microUSB connector we can mount on the back of the case (like this) - or maybe, I can just do a Coax to microUSB and give it power externally that way. Both of these solutions - just drill a hole in the back.

You could cut the power trace and put a DC in jack at the back of the speech case for the Pi. The only thing about the speech synthesizer is that it would have to go first in the chain to get power. The synth wouldn't pass any power anyway unless you've done the bypass mod, and paranoid folks could put a diode in place.

I really want to make a sidecar passthru one day so plug a PEB card like this into. I’m thinking of a way around the regulator bypass issue. We can put a switch in new home brew PEB cards that would bypass the regulator and let you use an external PSU. When bypassed, plugging it in the PEB would just make it a dead card because it’s not getting any power from the external Jack. It wouldn’t blow anything. When not bypassed, it goes through the regulator like normal and the DC in Jack wouldn’t matter and be bypassed. We can even have standard instructions to adapt regular cards for this so folks could use them plugged into the sidecar passthru. I’m sure the above idea has holes in it, but I’ve been mulling it over for a while....

They make them 3D printed now. These are notorious for cracking.
https://www.shapeways.com/product/CMBQ6D2Z4/alps-x-y-plotter-gear-qty-3-commodore-atari?li=shareProduct
Edit: These were the brass ones people were talking about at one time: http://www.xtreme-production.com/xtreme/index.php?main_page=product_info&cPath=159_307&products_id=3013

This might be something we can recreate the logic for to create reproduction CRU banked carts with current PAL/GALs. The CRU writes are pretty simple. We'd just have to figure out DBT's pinout on the cartridge (what they have hooked to where). I did pinout the TI Workshop cart at one time (earlier in this thread) - I have no idea why it had all almost all the cart address lines on it for the PAL, but I'm gathering it only manipulated three of them.

Yeppers! That's what I was referring to above. Ksarul has most of the copies of the /4A interface. We also have schematics for the Hexbus drive controller.
Edit: These are the only pictures I have of the interface. Ksarul will need to provide any higher resolution ones we need. He also has tons of the Hexbus interface chips he got as surplus. I do see a PAL. I do have the ROM dumped (attached).
hexbus_interface.BIN

I know Ksarul has a lot of the /4A Hexbus prototypes (and I have the ROM for one of them). But, has anyone thought about seeing if we can maybe put one of these in a sidecar for the JediMatt board? It would be kinda cool to use these to use Hexbus peripherals, connect to a CC-40 or TI-74, etc.
I'd be willing to partner with someone on it - but we'd need hires pics of the Hexbus sidecars in existence.